1. Field of the Invention
The present invention relates to a solid electrolytic capacitor using valve metal such as tantalum or niobium, and particularly relates to a solid electrolytic capacitor including a pair of lead terminals for mounting the capacitor on a mount object such as a printed circuit board.
2. Description of the Related Art
JP-A-2004-172527, for example, discloses a solid electrolytic capacitor to be mounted on a mount object such as a printed circuit board. For the convenience of explanation, the solid electrolytic capacitor disclosed in the document is shown in FIG. 7 of the present application, and generally indicated by reference sign 31. (The reference sign is added by the applicant of the present application.)
The solid electrolytic capacitor 31 includes a capacitor element 32 comprising a prismatic or columnar porous chip body 36 made by sintering valve metal powder such as tantalum or niobium powder and an anode bar 37 projecting from an end surface of the chip body 36. A dielectric film (not shown) having high insulation property is formed on the surfaces of the metal particles constituting the chip body 36, and a solid electrolytic layer (not shown) is formed on the dielectric film. Further, a cathode film 38 is formed on the solid electrolytic layer at the outer circumferential surface of the chip body 36. An anode lead terminal 33 is fixedly and electrically connected to the anode bar 37, whereas a cathode lead terminal 34 is fixedly and electrically connected to the cathode film 38 on the outer circumferential surface of the chip body 36. The capacitor element 32 is packaged in a generally prismatic mold member 35 made of a thermosetting synthetic resin so that the two lead terminals 33 and 34 are exposed at the bottom surface side. The lead terminals 33 and 34 are soldered onto a printed circuit board, for example.
The size of the mold member 35 is length L×width W×height (thickness) H=about 7.3 mm×about 4.3 mm×about 2.8 mm, and corresponds to the D case size of JIS. Thus, the mold member 35 of the related art is in the form of a generally rectangular parallelepiped (generally prismatic) whose length L is greater than the width W and the height H.
The mold member 35 includes a pair of end surfaces 35c and 35d extending in the width direction, at which the anode lead terminal 33 and the cathode lead terminal 34 are arranged, respectively. The capacitor element 32 is arranged between the lead terminals 33 and 34 so that the direction of projection of the anode bars 37 (orientation C0 of the capacitor element 32) corresponds to the longitudinal direction of the mold member 35 (See FIG. 7).
Recently, a solid electrolytic capacitor having a small size and a large capacitance is strongly demanded, and further, a decrease in not only the equivalent series resistance (ESR) but also in the equivalent series inductance (ESL) is demanded for improving the performance of a capacitor at a high frequency range. The equivalent series inductance increases as the length of the conductor through which current flows increases. On the other hand, the equivalent series resistance is generally inversely proportional to the surface area of a portion of the solid electrolytic layer which does not come into contact with the dielectric film (surface area of the non-contact portion). In the prior art solid electrolytic capacitor 31, the surface area of the non-contact portion is generally equal to the outer surface area of the chip body 36.
As will be understood from the above description, to reduce the equivalent series inductance of the solid electrolytic capacitor 31, the distance between the lead terminals 33 and 34 through which current flows need be shortened as much as possible. However, in the prior art solid electrolytic capacitor 31, the lead terminals 33 and 34 are arranged respectively at opposite end surfaces 35c and 35d extending in the width direction of the mold member 35 (i.e. spaced from each other in the longitudinal direction of the mold member 35), and the orientation c0 of the capacitor element 32 corresponds to the longitudinal direction of the mold member 35. With such an arrangement, the distance between the lead terminals 33 and 34 (distance through which current flows) becomes long, whereby the equivalent series inductance is relatively high.
When the distance between the lead terminals 33 and 34 is simply reduced to reduce the distance through which current flows, the overall length of the capacitor element 32 (the distance from the distal end of the anode bar 37 to the end surface of the chip body 36 which is opposite from the anode bar 37) cannot help being shortened, which results in the reduction of the outer dimension (volume) of the chip body 36. In such a case, the outer surface area of the chip body 36 decreases to increase the equivalent series resistance, and the capacitance of the capacitor element 32 decreases.
Moreover, since the outer dimensions of this kind of solid electrolytic capacitor 31 (size of the mold member 35) are standardized, the degree of freedom of design is small. Therefore, in the prior art structure in which the orientation c0 of the capacitor element 32 corresponds to the longitudinal direction of the mold member 35, it is difficult to reduce the distance between the lead terminals 33 and 34 while ensuring the sufficient volume of the chip body 36 of the capacitor element 32.